1. Field of the Invention
The present invention relates to a combustion state detecting apparatus for an internal-combustion engine, which apparatus controls ignition timing and the amount of fuel injection by detecting the combustion state of the internal-combustion engine by detecting the changes in the quantity of ions which are produced at the time of combustion in the internal-combustion engine and, more particularly, to a combustion state detecting apparatus for an internal-combustion engine, which apparatus is capable of detecting a misfire with high reliability to achieve optimum ignition timing without adding load to an electronic control unit, i.e. a microcomputer.
2. Description of Related Art
Generally, in an internal-combustion engine, the air and fuel, i.e. a fuel-air mixture, which has been introduced into the combustion chamber of each cylinder is compressed as a piston moves up, and high voltage is applied to a spark plug in the combustion chamber to generate an electric spark at the spark plug so as to burn the compressed fuel-air mixture; the explosive energy produced at that time is taken out as the force which pushes the piston down and it is converted to a rotary output.
When the combustion takes place in the combustion chamber in the foregoing combustion and expansion stroke, the molecules in the combustion chamber are ionized. Therefore, applying high voltage to the electrodes for detecting ionic current, which are installed in the combustion chamber, immediately after the combustion and expansion stroke causes ions with electric charges to move in the form of ionic current.
It is known that the ionic current sensitively reacts to the combustion state in the combustion chamber with a resultant change, making it possible to determine a combustion state such as a misfire or knocking in a cylinder by detecting the state of the ionic current, including the peak value thereof.
Based on the above, there has been proposed an apparatus which employs a spark plug as the electrodes for detecting ionic current to detect the combustion state, i.e. a misfire, of an internal-combustion engine according to the amount of ionic current detected immediately following ignition as described, for example, in Japanese Unexamined Patent Publication No. 2-104978.
FIG. 8 is a block diagram that schematically illustrates a conventional combustion state detecting apparatus for an internal-combustion engine; it shows an example wherein high voltage is distributed to spark plugs 8a through 8d of each cylinder via a distributor 7.
FIG. 9 is a timing chart illustrative of the operational waveforms of the voltage signals in FIG. 8; it shows the waveforms of ignition signal P, detection signal Ei of ionic current i, and ionic pulse Fi which are observed when normal combustion takes place.
In FIG. 8, a crankshaft of an internal-combustion engine, i.e. an engine, not shown, is provided with a crank angle sensor 1; the crank angle sensor 1 issues a crank angle signal SGT composed of pulses corresponding to engine speed.
The crank angle signal sGT is supplied to an electronic control unit (ECU) 2 constituted by a microcomputer and employed for various types of control arithmetic operations.
Each pulse edge of the crank angle signal SGT indicates the crank angle reference position of each cylinder, not shown, of the internal-combustion engine.
As shown in FIG. 9, for example, the rise edge of the crank angle signal SGT corresponds to a first reference position B75 degrees, which is 75 degrees before reaching compression upper dead center TDC and which provides the control reference for various control parameters including ignition timing of the internal-combustion engine, while the fall edge thereof corresponds to a second reference position B5 degrees in the vicinity of TDC, i.e. the initial ignition timing at the time of cranking.
The ECU 2 issues an ignition signal P for a power transistor TR driving an ignition coil 4, a fuel injection signal Q for an injector 5 of each cylinder, and driving signals for various actuators 6 including a throttle valve and ISC valve in accordance with the crank angle signal SGT received from the crank angle sensor 1 and the operational information received from various sensors 3 including a well-known intake sensor and a throttle opening sensor.
The ignition signal P issued from the ECU 2 is applied to the base of the power transistor TR to turn ON/OFF the power transistor TR.
The power transistor TR cuts off the supply of primary current i1 flowing into a primary winding 4a of the ignition coil 4 to boost primary voltage V1 so as to generate secondary voltage V2 of high voltage, e.g. a few tens of kilovolts, for ignition from a secondary winding 4b of the ignition coil 4.
A distributor 7 connected to the output terminal of the secondary winding 4b distributes and applies the secondary voltage V2 to spark plugs 8a through 8d in each cylinder so as to generate discharge sparks in the combustion chamber of the cylinder under ignition control, thereby burning a fuel-air mixture.
A series circuit comprised of a diode D1, a current limiting resistor R1, and current limiting zener diode DZ and diode D2 is provided between one end of the primary winding 4a and the ground to constitute a charging path for the biasing power supply, i.e. a capacitor to be discussed later, for detecting ionic current.
A capacitor 9 connected in parallel to both ends of the zener diode DZ is charged to a predetermined voltage by charging current in order to function as the power supply for detecting ionic current; it discharges immediately after ignition control to let ionic current i flow.
Diodes 11a through 11d provided between one end of the capacitor 9 and one end of the spark plugs 8a through 8d, and a resistor R2 inserted between the other end of the capacitor 9 and the ground make up, together with the capacitor 9, an ionic current detecting circuit through which the ionic current i flows.
The resistor R2 converts the ionic current i to a voltage to produce an ionic current detection signal Ei which is supplied to the ECU 2.
A pulse generating circuit 20 compares the ionic current detection signal Ei with a reference level Er shown in FIG. 9 to waveform-shape it into an ionic pulse signal Fi which includes the ionic pulse FP and supplies the ionic pulse signal Fi to the ECU 2.
The ECU 2 computes the control parameters for the internal-combustion engine and also detects the combustion state at the spark plugs 8a through 8d according to the ionic current detection signal Ei or the ionic pulse signal Fi to correct the control parameters.
Referring now to FIG. 9, the operation of the conventional combustion state detecting apparatus for an internal-combustion engine shown in FIG. 8 will be described.
First, the crank angle sensor 1 outputs the crank angle signal SGT according to the rotation of the internal-combustion engine. The ECU 2 outputs various driving signals including the ignition signal P for turning ON/OFF the power transistor TR according to the crank angle signal SGT indicative of the crank angle position of each cylinder and the operational state signals received from various sensors 3.
The power transistor TR turns ON when the ignition signal P is at high level and it allows the primary current i1 to flow through the primary winding 4a of the ignition coil 4; it cuts off the primary current i1 to the ignition coil 4 when the ignition signal P is switched from high to low level.
At this time, the primary voltage V1 is generated at the primary winding 4a due to counter electromotive voltage, thereby charging the capacitor 9 through a charging current path composed of the diode D1, the resistor R1, and the diode D2.
The charging of the capacitor 9 is completed when the charging voltage of the capacitor 9 becomes equal to the reverse breakdown voltage of the zener diode DZ.
When the primary voltage V1 appears at the primary winding 4a, the secondary winding 4b of the ignition coil 4 develops the secondary voltage V2 of a few tens of kilovolts; the secondary voltage V2 is applied to the spark plugs 8a through 8d of each cylinder via the distributor 7 so as to cause spark discharge to burn the fuel-air mixture.
When the fuel-air mixture burns, ions are produced in the combustion chamber of the cylinder, so that the ionic current i flows, the charging voltage of the capacitor 9 being the power supply.
For example, when the fuel-air mixture burns at the spark plug 8a, the ionic current i flows along a path composed of the capacitor 9, the diode 11a, the spark plug 8a, the ground, the resistor R2, and the capacitor 9 in the order in which they are listed. At this time, the resistor R2 converts the ionic current i to voltage so as to supply it as the ionic current detection signal Ei to the ECU 2.
The pulse generating circuit 20 applies the ionic current detection signal Ei as the ionic pulse signal Fi to the ECU 2.
The ECU 2 determines the combustion state in accordance with the ionic current detection signal Ei and the ionic pulse signal Fi; if, for example, it determines that a misfire has happened, then it cuts off the supply of fuel, or if it determines that knocking has occurred, then it delays the ignition timing to restrain the knocking.
Thus, the combustion state is reflected on the control parameters, namely, the ignition signal P and the fuel injection signal Q, to optimize the ignition timing or the control amount of the fuel injection, etc. so as to provide optimum, maximum engine output torque.
However, at the rise timing and the fall timing of the ignition signal P, i.e. at the time of energizing and de-energizing the ignition coil 4, an instantaneous noise signal En shown in FIG. 9 is superimposed on the ionic current detection signal Ei.
The noise signal En directly turns into a noise pulse Fn and it is supplied as the ionic pulse signal Fi to the ECU 2.
Therefore, the ECU 2 may erroneously determine the combustion state because of the noise pulse Fn.
Thus, the conventional combustion state detecting apparatus for an internal-combustion engine has been posing a problem in that, although it determines the combustion state according to the ionic current i, it provides no effective measures against the noise signal En and the like superimposed on the ionic current detection signal Ei at the time of ignition control, making it impossible to accurately detect the combustion state in the internal-combustion engine.
There has been another problem in that setting an effective period of the ionic pulse signal Fi during the arithmetic processing performed by the ECU 2 adds load to the ECU 2 implementing the arithmetic processing, thus adversely affecting the controlling operation, which is the major function of the ECU 2.